Casanova crabs make waves by rejecting the group rhythm

When faced with a troupe of hopeful males dancing in time to win her attention, the female fiddler crab is surprisingly indifferent. Instead, new research shows that females are drawn to males who lead the group’s beat… and the ones who rebel against it.

Male fiddler crabs sport one outrageously oversized claw, which they wave rhythmically to attract a mate. Groups of crabs on the beach often appear to wave in synchrony, but it seems that keeping in time with your neighbours is not a skill that excites the ladies. Researchers from the Australian National University deciphered the secrets to alluring dance moves by using robotic crabs that waved with differing levels of synchrony, and recording the reaction of female onlookers. They found that the most successful males either began each wave slightly ahead of the other crabs, or waved on the opposite beat. Those who went with the crowd or lagged slightly behind were far less popular.

The appeal of leaders and rebels appears to lie in when their wave begins. Unlike lagging or synchronising males, the start of a popular male’s wave does not overlap with the movements made by the rest of the crowd. This helps him catch the female’s eye, ensuring she concentrates on his wave without getting distracted by his neighbours. So why do most males end up going with the flow and losing out on a partner? It’s likely that males decide when to wave by watching a neighbour and waving immediately after him, to avoid overlapping. However, when a large number of crabs use this strategy, it backfires, increasing the likelihood that many of them will accidentally synchronise. So whilst synchronisation dampens each male’s chances, the very behaviours they have evolved to prevent it have unintentionally made it the norm.

Carrying out sexual displays in synchrony isn’t always a bad move: some species have evolved a slick choreography with their neighbours to attract attention. For example, female fireflies are drawn to groups of males who flicker in harmony with each other, and male wolf spiders coordinate their leg-drumming displays to maximise their sex appeal.  This is a neat example of how similar-looking behaviours can arise with very different backgrounds: in this case, fierce competition and smart cooperation have both given rise to some very smooth beat-keeping in the animal kingdom.

Here's a video of the robot crabs in action during another study:

The original paper appears in December 2014's issue of the journal Animal Behaviour, which can be found here.

Video credit: Science/AAAS
Picture credit: Sophie Callander/Australian National University